An image sensor may be a semiconductor device that is configured to convert optical images into electrical signals. Examples of image sensors may include charge coupled device (CCD) type and complementary metal oxide semiconductor (CMOS) type. The CMOS type image sensor is abbreviated to CIS (CMOS image sensor). The CIS has a plurality of two-dimensionally arranged pixels. Each of the pixels includes a photodiode. The photodiode serves to convert incident light into electrical signals.